Screen dimming problem on Samsung crt monitor

Hey dudes,
How are you all? I am getting a serious problem with my Samsung monitor. When I start the pc it runs very fine. But, approximately after 15 minutes it becomes so much dim. It becomes too much intend that I can’t read the texts correctly. But, when I restart the pc it becomes normal for a long time.
I don’t know what sort of problem is this? Can anybody please help me to solve this problem permanently?
Thank you very much
